Wierd Gym Attacked

Sometimes trainers will do this in my area too! I think it's to maximize the resources and time you have to spend to prestige the gym back up. It might be a "mind games" type thing where they eventually hope to exhaust your efforts and then take over. Only reason I say that is because most of the competitive aspect in-game is fighting over territory (gyms) and it's a relatively meta thing to do.

I have a similar mentality where I'll check on the gyms i'm defending periodically. If someone dropped it from let's say 52,000 to 37,000 prestige then left, I'll go train it back to 50,000. The more I do that, the less other people seem to go after the gym(s).

Short answer: hackers.

They move from one area to the other, and sometimes I think they don't put mon just because they have enough coins and don't want to be reported, if you take down a level 7-10 gym to zero and put a mon inmediately after people can suspect that you are a hacker and try to get you in the middle of the work, if you leave it at 1 or grey and someone else comes and put a mon and you put it after few people will suspect.

I have seen this in my town, each month we have a new wave of hackers coming and taking everything (there are 11 gyms) at one time a day, for example 2 in the morning, eventually they leave for some weeks but they always come back...

We have a gym in the middle of a bridge, I prestiged it up to 8 and wait for 2 hours there, someone start tooking it down after a while, it couldn't be anyone there unless they were swimming in the middle of the river...I reported 2 times, this player is still up and running after those reports.

They're just spoofers. They don't want to put a Pokemon in bc they don't want to be reported.

Someone has a grudge against a player in those gyms.

The first gym I dropped into was empty, left by someone.
I'll leave a gym empty every once in a while to hopefully get a new comer interested in gym play.

I bet they are cheaters.
Sad

I do the same thing against the spoofers in my area. The reason I dont leave mons is because I dont want them to know who I am and thus retaliate against me. Also no point in dropping off a mon if the spoofer will just knock out my one mon and have 3 spaces open to add their alts. By crippling the gym and making it harder to prestige I force the spoofer to attack their own gym(which means building up a separate account with mons and counters) or to train up a gym against a Snorlax or Lapras. Obviously since they spoof just knocking down and claiming it does nothing since the spoofer doesnt have to drive out to take it back.
Cant really beat somebody in this game. There is no winning against another player. So i try to create a time differential. For every hour I spend I try to froce them to spend 2 hours and for me thats as close to a win as I can get.

There is something happening here that you may not understand as I am a trainer that does exactly what you are describing.

At any given time I am defending 20 gyms, normally my defenders are either the top defender or in the top 5 so they sit there for months until a rival team clears the entire gym.

Now by attacking other gyms even though I already have 20 defenders, what I am doing is keeping rivals busy. They have to retake their gyms, prestige up their gyms after I knock it back down to level 1. Or I just got sick of seeing the gym at level 10 for weeks with the same trainers in all the surrounding gyms.

Also I battle in a style of attrition. I make them waste, revives and potions when I knock out their defenders. while I keep a huge supply of 100+ Max revives and 700+ Max potions because I hardly use them unless I need to restore a Chansey lol.